1. Field of the Invention
The present invention generally relates to a commutation controller for an electronically commutated electrical machine and, in particular, to such a control system for a switched reluctance machine.
2. Description of Related Art
In general, a reluctance machine is an electrical machine in which torque is produced by the tendency of its movable part to move into a position where the reluctance of a magnetic circuit is minimized, i.e. where the inductance of the exciting winding is maximized. In one type of reluctance machine, the energization of the phase windings occurs at a controlled frequency. This type is generally referred to as a synchronous reluctance machine. It may be operated as a motor or a generator. In a second type of reluctance machine, circuitry is provided for detecting the angular position of the rotor and energizing the phase windings as a function of the rotor position. This second type of reluctance machine is generally known as a switched reluctance machine. It too may also be a motor or a generator. The characteristics of such switched reluctance machines are well known and are described in, for example, "The characteristics, design and application of switched reluctance motors and drives" by Stephenson and Blake, PCIM'93, Nurnberg, Jun. 21-24, 1993, which is incorporated herein by reference.
FIG. 1 shows the principal components of a typical switched reluctance system 10. The input DC power supply 11 can be either a battery or rectified and filtered AC mains. The DC voltage provided by the power supply 11 is switched across the phase windings 12 of the machine 16 by a power converter 13 under the control of an electronic control unit 14. The switching must be correctly synchronized to the angle of rotation of the rotor for proper operation of the switched reluctance system 10. A rotor position detector 15 is typically employed to supply signals corresponding to the angular position of the rotor. The output of the rotor position detector 15 may also be used to generate a speed feedback signal.
The rotor position detector 15 may take many forms, for example it may take the form of hardware, as shown schematically in FIG. 1, or of a software algorithm which calculates the position from other monitored parameters of the drive system, for example as described in EP-A-0573198 (Ray), which is incorporated herein by reference. In some systems, the rotor position detector 15 can comprise a rotor position transducer that provides output signals that change state each time the rotor rotates to a position where a different switching arrangement of the devices in the power converter 13 is required.
Timing of the energization of the phase windings in a switched reluctance machine depends heavily on accurate detection of the angular position of the rotor. The importance of accurate signals from the rotor position transducer 15 may be explained by reference to FIGS. 2 and 3, which explain the switching of a reluctance machine operating as a motor.
FIG. 2 generally shows a rotor pole 20 approaching a stator pole 21 according to arrow 22. As illustrated in FIG. 2, a portion 23 of a complete phase winding 12 is wound around the stator pole 21. As discussed above, when the portion of the phase winding 23 around stator pole 21 is energized, a force will be exerted on the rotor, tending to pull rotor pole 20 into alignment with stator pole 21. FIG. 3 generally shows typical switching circuitry in the power converter 13 that controls the energization of the portion 23 of the phase winding 12 around stator pole 21. The circuit comprises first and second switches 31/31 and a return diode 33 and 34 for each switch. When the switches 31 and 32 are closed, phase winding 12 is coupled to the source of DC power and the phase winding is energized. Many other configurations of switching circuitry are known in the art: some of these are discussed in the Stephenson & Blake paper cited above.
In general, the phase winding is energized to effect rotation of the rotor as follows. At a first angular position of the rotor (called the turn-on angle, .theta..sub.ON) the controller 14 provides switching signals to turn on both switching devices 31 and 32. When the switching devices 31 and 32 are on, the phase winding is coupled to the DC supply, causing an increasing magnetic flux to be established in the machine. It is this magnetic flux acting on the rotor pole that produces the motoring torque. As the magnetic flux in the machine increases, current flows from the DC supply through the switches 31 and 32 and the phase winding 12. In some controllers, current feedback is employed and the magnitude of the phase current is controlled by chopping the current by rapidly switching one or both of switching devices 31 and/or 32 on and off. In motoring operation, the turn-on angle is often chosen to be the rotor position where the center-line of an inter-polar space on the rotor is aligned with the centerline of a stator pole, but it may be some other angle.
In many systems, the phase winding remains connected to the DC bus (or connected with chopping if chopping is employed) until the rotor rotates such that it reaches what is referred to as the "freewheeling angle", .theta..sub.FW. When the rotor reaches an angular position corresponding to the freewheeling angle (e.g., the position shown in FIG. 2) one of the switches, for example 31, is turned off. Consequently, the current flowing through the phase winding will continue to flow, but will now flow through only one of the switches (in this example 32) and through only one of the return diodes (in this example 34). During the freewheeling period, the voltage drop across the phase winding is small, and the flux remains substantially constant. The circuit remains in this freewheeling condition until the rotor rotates to an angular position known as the "turnoff angle", .theta..sub.OFF, (e.g. when the centerline of the rotor pole is aligned with that of the stator pole). As with the turn-on angle, the turn-off angle may be chosen to be at some other position.
When the rotor reaches the turnoff angle, both switches 31 and 32 are turned off and the current in phase winding 12 begins to flow through the diodes 33 and 34. The diodes 33 and 34 then apply the DC voltage from the DC bus in the opposite sense, causing the magnetic flux in the machine (and therefore the phase current) to decrease.
FIG. 4(a) shows a typical current waveform in the chopping mode of operation, where the current is chopped between two fixed levels. As the speed of the machine rises, there is less time for the current to rise to the chopping level, and the drive is normally run in a "single-pulse" mode of operation. In this mode, the turn-on, freewheel and turn-off angles are chosen as a function of, for example, speed and load torque. FIG. 4(b) shows a typical single-pulse current waveform where the freewheel angle is zero.
It is well known that the values of turn-on, freewheel and turn-off angles for different motor speeds can be predetermined and stored in some suitable format, such as a look-up table, for retrieval by the control system as required, or can be calculated or deduced in real time. In either case, there is a requirement for the appropriate angular position of the rotor to be detected, so that the required switching action can be carried out. In general, this requires either that the position detector itself is sufficiently sophisticated to produce fine resolution of position or that the rotor position detector signals can be interpolated in some fashion to provide position information at fine resolution. The first case involves the use of a relatively complex and expensive position encoder. One example of the second case is described in EP-A-0735664 (Sugden), which is incorporated herein by reference and which uses a high-frequency clock to generate a digital ramp, from which the appropriate angular positions can be interpolated.
In FIG. 5, a rotor position transducer (RPT) 15 is illustrated which uses optical sensors 42 cooperating with a moving vane 44 which has light-interrupting angularly spaced segments on it. The vane 44 rotates with the rotor of the electrical machine, as shown schematically in FIG. 1, causing the sensors to produce high and low outputs as the segments pass each sensor. It will be appreciated that, in this example, the output of each sensor is used to control the energization of a respective phase of the machine winding. Other arrangements are known where the number of sensors is greater or smaller than the number of phases.
The controller 14 of FIG. 1 can receive a set of output signals from the RPT and utilize those signals to control the switching of power devices to control the energization of the phase windings of the switched reluctance machine. It does this by observing the pulse trains generated by the sensors 42 of FIG. 5 as the vane 44 rotates. FIG. 6 shows the signals produced by the three sensors 42 of FIG. 5 as the vane 44 rotates at constant speed. FIG. 6 also shows a signal produced by simple logic gating of the three sensor signals so that a rising or falling edge is produced each time there is a transition in one of the three signals.
FIG. 7(a) shows the idealized inductance waveform of one phase (e.g. Phase A) of the machine shown in FIG. 1. It will be recognized by those familiar with reluctance machines that it is periodic, with the period defined by the rotor pole pitch. In practice, the corners of the inductance waveform are rounded by second-order effects, such as flux fringing, but such influences are secondary to the operation of the machine. FIG. 7(b) shows the relationship of the signal RPT.sub.A of FIG. 6 to the inductance waveform. The sensor 42 has been aligned relative to the stator poles (and the rotating vane has been aligned relative to the rotor poles) such that it provides transitions in output signal level at the centers of the maximum and minimum inductance regions. These transitions can be used, e.g. as described in the aforementioned EP-A-0735664 (Sugden), to control the timing of the commutation of an electronically commutated machine. If the control angles .theta..sub.ON and .theta..sub.OFF are specified in terms of voltage levels or digital words, as appropriate, then detection of the points at which the appropriate levels or words are reached can be used to generate the required firing signals for the switching devices, as will now be described.
FIG. 7(c) illustrates a motoring ramp for phase A, comprising a voltage level which rises linearly with rotor angle. The ramp resets at each falling edge of the RPT signal. Two thresholds are set corresponding to the angles .theta..sub.ON and .theta..sub.OFF at which the phase is energized and, thereafter, de-energized. .theta..sub.ON causes the start of a firing pulse when the ramp passes the .theta..sub.ON threshold. The firing pulse is maintain ed until the same ramp passes the .theta..sub.OFF threshold. By adjusting the .theta..sub.ON threshold, the moment of initiation of the motoring firing pulse is varied. Similarly, by adjusting the .theta..sub.OFF threshold, the duration of the firing pulse is varied. FIG. 7(d) illustrates a motoring firing pulse which can be seen to be generated by the transition of the ramp past the .theta..sub.ON and .theta..sub.OFF thresholds.
FIG. 7(e) illustrates the ramps initiated in a generating mode at the rising edge of the RPT signal. Again, the transition of the ramp past the .theta..sub.ON and .theta..sub.OFF thresholds determines the start and duration of the generating firing pulses. The pulses are shown in FIG. 7(f).
Systems based on this principal are economical in components and work well in steady state. However, when a transition is made from motoring to generating or vice versa, there is a likelihood of an initially misplaced firing pulse. This can lead to a randomly positioned torque pulse in the machine which can cause acoustic noise and lead to high fault currents. One option is to detect a situation in which a firing pulse will be misplaced in the transition from one mode to the other and to omit that pulse altogether. Control in the selected mode is then only taken up in the next phase in the sequence. The consequence of this is a significant disturbance of the torque due to the temporary prolonged absence of any phase energization at all.
An alternative approach is to inhibit the ramp associated with the initial mode and to start the first ramp of the second mode at the moment of transition between modes. This is illustrated in FIG. 7(g). At angular position T, the command to make the transition from motoring to generating modes cancels the motoring ramp which is immediately succeeded by the generating ramp. However, because the transition occurs between the notionally correct start of the generating ramp and the end of the notionally correct end of the generating firing pulse, the generating ramp does not start in the correct place coincident with the rising edge of the RPT signal. As shown in FIG. 7(h), the result is that the first generating firing pulse is delayed and of a shorter duration due to the error in the magnitude of the ramp at a given rotor position. This is to be contrasted with the desired position and duration of the first generating firing pulse following the transition shown in FIG. 7(i).
It is an object of the present invention to provide a cost-effective method of deriving angular position information under both steady-state and transient conditions and producing reliable firing pulses for an electronically commutated electrical machine.